This invention relates to a process for making polyolefins. In particular, the invention relates to a polymerization process with a catalyst precursor, an activator, and an organosilane modifier. The modifier increases polyolefin molecular weight without significantly reducing catalyst activity.
Organosilane modifiers have been studied with Ziegler-Natta catalysts. Y. V. Kissin, J. Polym. Sci. Part A: Polym. Chem., 33 (1995) 227, reports a series of ethylene-hexene copolymerization experiments with varying amounts of diphenyldimethoxysilane. The silane increases the Mw and decreases the catalyst activity. M. Harkonen, J. V. Seppala and T. Vaananen, Makromol. Chem. 192 (1991) 721, studied Ziegler-Natta catalyzed propylene polymerizations with fourteen alkoxysilanes. In each case, the activity of the catalyst decreased (18-66%) versus the control experiment without organosilane. In all but two instances, the viscosity-average molar mass increased.
Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of xcex1-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes commonly include one or more cyclopentadienyl groups or cyclopentadienyl-like groups such as indenyl, fluorenyl, and substituted varieties of these, but many other ligands have been used. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. Other known polymerization-stable ligands are heteroatomic ligands such as boraaryl, pyrrolyl, indolyl, indenoindolyl, quinolinoxy, pyridinoxy, and azaborolinyl as described in U.S. Pat. Nos. 5,554,775, 5,539,124, 5,637,660, 5,902,866 and 6,232,260.
The incorporation of hydrosilanes in polymerizations using cyclopentadienyl metallocene catalysts is described in EP 0739910 A2, J. Am. Chem. Soc. 121 (1999) 8791, and in U.S. Pat. Nos. 5,578,690, 6,075,103 and 6,077,919. High levels of hydrosilane are used to lower the polymer molecular weight. For instance, in EP 0739910A2, 0.8 to 10.6 mmoles of silane are used per 0.029 mmoles cyclopentadienyl metallocene catalyst. At these levels, the polymer molecular weight decreases with increasing hydrosilane.
One attribute of many metallocene and single-site catalysts is their propensity to produce lower molecular weight polymers. Thus for certain commercial applications, increased molecular weight is desirable. For instance, polymer toughness and strength generally increase with increased molecular weight. Despite the importance of olefin polymerizations and the considerable research that has been done on various catalyst systems, there remains a need to modify the catalyst to be able to increase the molecular weight of the resultant polyolefin. Surprisingly, we have found that low levels of organosilanes can be used to increase polyolefin molecular weight without significantly reducing catalyst activity.
This invention is a process for the polymerization of an olefin. An olefin is polymerized with a catalyst precursor in the presence of an activator and an organosilane modifier. The organosilane allows the catalyst to maintain high activity while increasing polyolefin molecular weight. The process is robust, easy to practice and affords polyolefins with improved properties.
The invention is a process for polymerizing olefins. Suitable olefins are C2-C20 xcex1-olefins, such as ethylene, propylene, 1-butene, 1-hexene, 1-octene and mixtures thereof. Preferred olefins are ethylene, propylene and mixtures thereof with xcex1-olefins such as 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.
The polymerization is performed with a catalyst precursor comprising a Group 3 to 10 transition or lanthanide metal, M, and at least one polymerization-stable, anionic ligand. Examples of suitable anionic ligands include substituted and unsubstituted cyclopentadienyl, fluorenyl, and indenyl, or the like, such as those described in U.S. Pat. Nos. 4,791,180 and 4,752,597, the teachings of which are incorporated herein by reference. A preferred group of polymerization-stable ligands are heteroatomic ligands such as boraaryl, pyrrolyl, indolyl, indenoindolyl, quinolinoxy, pyridinoxy, and azaborolinyl as described in U.S. Pat. Nos. 5,554,775, 5,539,124, 5,637,660, 5,902,866 and 6,232,260, the teachings of which are incorporated herein by reference. Complexes that incorporate indenoindolyl ligands are particularly preferred (see U.S. Pat. No. 6,232,260 and PCT Int. Appl. WO 99/24446).
The catalyst precursor also usually includes one or more labile ligands such as halides, alkyls, alkaryls, aryls, dialkylaminos, or the like. Particularly preferred are halides, alkyls, and alkaryls (e.g., chloride, methyl, benzyl).
The polymerization-stable ligands can be bridged. Groups that can be used to bridge the ligands include, for example, substituted or unsubstituted methylene, ethylene, 1,2-phenylene, and dialkyl silyls. Normally, only a single bridge is included. Bridging changes the geometry around the transition or lanthanide metal and can improve catalyst activity and other properties such as comonomer incorporation.
Exemplary catalyst precursors are bis(cyclopentadienyl)zirconium dimethyl, bis(cyclopentadienyl)zirconium dichloride, bis(indenyl)titanium dibenzyl, cyclopentadienyl(indenoindolyl)zirconium dichloride, bis(fluorenyl) zirconium dimethyl, 8-quinolinoxy(cyclopentadienyl)titanium dimethyl, bis(2-pyridinoxy)titanium diethyl, (1-dimethylaminoborabenzene)cyclopentadienyl-zirconium dichloride, bis(1-methylborabenzene)zirconium dimethyl, bis(indolyl)zirconium dimethyl, and the like.
Preferred transition metals are Group 4-6 transition metals and of these zirconium is especially preferred.
The process is performed in the presence of an activator. Suitable activators ionize the catalyst precursor to produce an active olefin polymerization catalyst. Suitable activators are well known in the art. Examples include alumoxanes (methyl alumoxane (MAO), PMAO, ethyl alumoxane, diisobutyl alumoxane), alkylaluminum compounds (triethylaluminum, diethyl aluminum chloride, trimethylaluminum, triisobutyl aluminum), and the like. Suitable activators include acid salts that contain non-nucleophilic anions. These compounds generally consist of bulky ligands attached to boron or aluminum. Examples include lithium tetrakis(pentafluorophenyl)borate, lithium tetrakis(pentafluorophenyl)-aluminate, anilinium tetrakis(pentafluorophenyl)borate, and the like. Suitable activators also include organoboranes, which include boron and one or more alkyl, aryl, or aralkyl groups. Suitable activators include substituted and unsubstituted trialkyl and triarylboranes such as tris(penta-fluorophenyl)borane, triphenylborane, tri-n-octylborane, and the like. These and other suitable boron-containing activators are described in U.S. Pat. Nos. 5,153,157, 5,198,401, and 5,241,025, the teachings of which are incorporated herein by reference.
Suitable activators also include aluminoboronatesxe2x80x94reaction products of alkyl aluminum compounds and organoboronic acidsxe2x80x94as described in U.S. Pat. Nos. 5,414,180 and 5,648,440, the teachings of which are incorporated herein by reference.
The amount of activator needed relative to the amount of catalyst precursor depends on many factors, including the nature of the catalyst precursor and activator, the desired reaction rate, the kind of polyolefin product, the reaction conditions, and other factors. Generally, however, when the activator is an alumoxane or an alkyl aluminum compound, the amount used will be within the range of about 0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles, of aluminum per mole of M. When the activator is an organoborane or an ionic borate or aluminate, the amount used will be within the range of about 0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles, of activator per mole of M.
The activator is normally added to the reaction mixture at the start of the polymerization. However, when a supported catalyst system is used, the activator can be deposited onto the support along with the catalyst precursor.
The catalyst precursor and activator are optionally used with an inorganic solid or organic polymer support. Suitable inorganic supports include silica, alumina, silica-aluminas, magnesia, titania, clays, zeolites, or the like. The inorganic support is preferably treated thermally, chemically, or both prior to use to reduce the concentration of surface hydroxyl groups. Thermal treatment consists of heating (or xe2x80x9ccalciningxe2x80x9d) the support in a dry atmosphere at elevated temperature, preferably greater than about 100xc2x0 C., and more preferably from about 150xc2x0 C. to about 600xc2x0 C., prior to use. A variety of different chemical treatments can be used, including reaction with organo-aluminum, -magnesium, -silicon, or -boron compounds. See, for example, the techniques described in U.S. Pat. No. 6,211,311, the teachings of which are incorporated herein by reference. Suitable organic polymer supports include polyethylene, polypropylene and polystyrene. Small particle size density polyethylene has been found to be particularly suitable. The catalyst precursor and activator can be deposited on the support in any desired manner. For instance, the components can be dissolved in a solvent, combined with a support, and stripped. Alternatively, an incipient-wetness technique can be used. Moreover, the support can simply be introduced into the reactor separately from the precursor and activator.
The loading of catalyst precursor on the support varies depending upon a number of factors, including the identities of the precursor and the support, the type of olefin polymerization process used, the reaction conditions, and other concerns. Usually, the amount of precursor used is within the range of about 0.01 to about 10 wt. % of transition metal based on the amount of supported precursor. A more preferred range is from about 0.1 to about 4 wt. %.
The modifier is an organosilane. By xe2x80x9corganosilane,xe2x80x9d we do not include hydrosilanes. Preferred organosilane modifiers are alkyl alkoxysilanes which have the formula RySi(OR)4xe2x88x92y where each R is independently C1 to C30 hydrocarbyl and y is an integer from 0 to 3. More preferably, the organosilane modifiers are dialkyl dialkoxysilanes. Exemplary dialkyl dialkoxysilanes are diisopropyldimethoxysilane, isobutylisopropyl-dimethoxysilane, di-s-butyidimethoxysilane, cyclohexyl-methyldimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, dicyclopentyldimethoxysilane, and mixtures thereof.
Surprisingly, very low levels of modifier are effective. Preferably, the modifier is added at a level of from 0.1 to 10 moles per mole of catalyst precursor, more preferably from 0.2 to 2 moles per mole of catalyst precursor. Lower levels are often ineffective. Excessively higher levels do not result in further increases in molecular weight and may be detrimental to catalyst activity.
Preferably, the modifier is added to the reactor at around the same to time as the addition of the catalyst precursor. The addition can be prior to or subsequent to the addition of the catalyst precursor to the reactor. Optionally, the modifier is premixed with the catalyst precursor and they are fed together to the reactor. The modifier can be added neat or as a solution. When the modifier is added as a solution, suitable solvents include alkanes such as hexane or isobutane. Optionally, the solvent is an olefin used for the polymerization.
The modifiers can be used in a variety of well-known olefin-polymerization processes, including gas, high pressure liquid, slurry, solution, or suspension-phase techniques and combinations of these. The pressures used typically range from about 15 psig to about 30,000 psig. Polymerization temperatures range from about xe2x88x92100xc2x0 C. to about 300xc2x0 C., more preferably from about 20xc2x0 C. to about 200xc2x0 C., and most preferably from about 60xc2x0 C. to about 150xc2x0 C.